In the following, a number of terms will be first defined to which reference will later be made.
The visual acuity or visus V is the extent of the capability of a living being to perceive patterns and contours as such in the outer world with the being's viewing organ. The dimensionless characteristic visus V is defined asV=1′/(individual angular visual acuity)  (1)wherein the angular visual acuity is the resolution power with which two viewing objects can still be perceived as separate (so-called minimum “separabile”).
The depth of field or imaging depth T of the eye is, according to DIN 19040, 5-3.12, the magnitude of the region forward and rearward of the set point AE whereat the object points are imaged with a non-perceptible unsharpness of the retina image. The imaging depth T of the eye is dependent upon the pupil diameter and upon the central visual acuity. For a pupil diameter of 2.9 mm and a visual acuity V of 1.0, the imaging depth T results, theoretically, from the distances corresponding to 0.1 dpt on both sides of the set point AE: 10 cm in the light direction behind the set point, 10 cm in light direction forward of the set point results in an imaging depth of 20 cm which corresponds to approximately 0.25 dpt (source: Helmut Goersch, Lexicon of Optometry, 2nd edition 2001).
Use distance ause is the distance, which is selected by a test person, from the eye principal point HA to the object.
The term addition Add or near addition is understood to mean the difference between the spherical effect of the near portion and that of the remote portion of a continuous vision lens or a multifocal spectacle lens.
Accommodation (from the Latin: accomodare “adapt, put on, fixing”) characterizes operations for changing the light path within the eye in order to sharply image objects at different distances in the retina plane. In mammals and birds, the form of the elastic lens is changed in accordance with a theory of Helmholtz for accommodation in order to vary the refractive power. A theory of Schachar additionally proceeds from a forward displacement of the eye lens during the accommodation.
At the same time as the tensioning of the ciliary muscle, the eyes carry out a convergence movement, that is, the eyes move toward each other so that the viewing axes intersect at the fixation point. The convergence movement is, inter alia, a condition precedent for the fusion of the visual impressions of both eyes close in. Furthermore, a pupil constriction (convergence myosis) automatically occurs. The three coupled reactions during the adjustment of the eye on a near object (accommodation, convergence and synergistic pupil reflex) are characterized as accommodation triad or convergence reaction.
The far point refraction AR is the reciprocal of the far point distance aR, that is, the distance aobject of the location of the object from the object side principal point HA of the eye wherein the eye sharply perceives the object without accommodation. The adjusting point refraction AE is the reciprocal of the focusing point distance aE (also called: accommodation distance), that is, the distance aobject of the object point, which, in the instantaneous accommodation state, is imaged in the fovea and is therefore sharply seen.
The accommodation-caused change of the focusing point refraction AE per unit of time is characterized as the accommodation speed VA in dpt/s. The accommodation speed VA results from the ratio of the change of the focusing point refraction AE to the accommodation time tA. It is dependent upon the age and the focusing point refraction AEV, AEn before and after the accommodation and amounts to approximately 2 to 5 dpt/s.
The accommodation requirement A, which is given in diopters (dpt) is understood, in accordance with DIN 5340-15, to be the reciprocal of the object distance aobject (in meters) from the object side principal point HA of the eye. If an optical system is disposed forward of the eye, then the distance of the image developed by this system takes the place of the object distance aobject.
The relative accommodation ΔArel is defined in DIN 5340-12 as the change of the amplitude of accommodation ΔA for unchanged vergency position and unchanged sharp binocular normal vision. The change of the amplitude of accommodation ΔA is forcibly achieved by lenses having a spherical effect.
Mental accommodation ΔApsych (proximal accommodation=instrument myopia) is understood to be the near accommodation Anear triggered by real objects because of the consciousness of the nearness.
The accommodation stimulus ΔAstimulus is the stimulus for accommodation which is triggered by a change of the fixation distance aobject by the retina image which is at first unsharp.
The amplitude of accommodation ΔA, which is likewise given in diopters (dpt), is understood according to DIN 5340-99 to be the difference between the far point refraction AR and the set point refraction AE. The term amplitude of accommodation ΔA is also known as external accommodation.
The DIN 5340-20 defines the maximum amplitude of accommodation or the maximum amplitude of accommodation ΔAmax as the difference between the far point refraction AR and the near point refraction Ap. The near point refraction Ap is the reciprocal of the distance ap of the eye principal point HA to the sharply viewed object point with strongest accommodation.
For infants, the maximum amplitude of accommodation ΔAmax is approximately 14 dpt. Referred to the total refractive power of the eye of approximately 58 dpt, this corresponds to a variation of approximately 25%. At an old age, the maximum amplitude of accommodation ΔAmax drops to values under 2 dpt or 4%. For this reason, the smallest distance, that is, the near point ap, wherein objects can still be sharply seen is magnified from approximately ap=7 cm for infants to more than ap=50 cm in persons of old age.
An exact separation between maximum amplitude of accommodation ΔAmax and depth of field T of the human eye is to date not possible. The sum from actual maximum amplitude of accommodation ΔAmax, which is defined by the above-mentioned mechanisms of the accommodation triad and the depth of field T is therefore referred to hereinafter as physiological maximum amplitude of accommodation ΔA*max.
In 1922, Duane determined the age dependency of the average physiological maximum amplitude of accommodation ΔA*max,m from the viewing impressions of 5,000 test persons having normal vision. In the determination of this data, no distinction was drawn between the maximum amplitude of accommodation ΔAmax and the depth of field T, that is, the determined curve is a superposition of both effects. The middle curve 802 of the diagram of FIG. 15 shows this age dependency of the average physiological maximum amplitude of accommodation ΔA*max,m. The upper and the lower curves (804, 806) represent the respective physiological limits of the dispersion. The diagram of FIG. 16 shows the corresponding age dependency of the minimum visual distance ap.
The reason for the reduction of the maximum amplitude of accommodation ΔAmax is a decreased elasticity of the lens capsule or a lens thickening because of lifelong growth of the lens shell (Helmholtz theory). The elasticity decreases with increasing age. It was determined that even for vanishing elasticity of the lens capsule, a residual of accommodation amplitude ΔAmax remains. The so-called Schachar theory according to which, additionally, one proceeds from a forward displacement of the eye lens during the accommodation can explain the remaining residual of maximum amplitude of accommodation ΔAmax which is not lost with increasing age.
If the maximum amplitude of accommodation ΔAmax including the depth of field T drops, with increasing age (see FIG. 15) below approximately 3 diopters (the newspaper must be held at a distance of over 35 cm for reading with distance spectacles), one speaks of presbyopia. Simple reading spectacles, bifocal spectacles, continuous vision spectacles or multifocal contact lenses or intraocular lenses can compensate for the presbyopia.
The term use accommodation ΔAuse is understood to be that accommodation A which can be provided without difficulty over a longer time span. It amounts to approximately ½ (Reiner) to ⅔ (Schober) of the maximum amplitude of accommodation ΔAmax.
From the state of the art, a great number of methods are known for determining an addition Add of multifocal or continuous vision spectacle lens, contact lens or intraocular lens with this addition Add being suitable for an ametropic person and being adapted to that person's requirements. All these methods have in common that the addition Add should not exceed the reciprocal value of the minimum use distance ause,min. The methods, however, differ from each other in the determination of the actual value of the addition Add. This subject matter is explained in the following based on a simple computation example.
It is assumed that a test person experiences as pleasant to hold a document at a distance of 40 cm to the eye when reading. This distance is identified hereinafter as the use distance ause. Furthermore, it is assumed that the minimum use distance ause,min at which the test person holds the reading material is 33 cm at any time. The reciprocal of the minimum use distance ause,min amounts to 3 dpt. These 3 dpt define the accommodation requirement A for the minimum use distance ause,min.
If the test person were, for example, 50 years old, then the test person would have a residual maximum amplitude of accommodation ΔAmax of approximately 2 dpt according to the diagram of Duane of FIG. 15. Because the actually used accommodation effort, that is, the use accommodation ΔAuse according to the theory of Reiner is approximately half (according to the theory of Schober approximately two thirds) of the still remaining accommodation amplitude ΔAmax, the test person would actually use approximately 1 dpt (up to 1.5 dpt according to Schober) of his or her maximum amplitude of accommodation ΔAmax. The correct addition Add for a spectacle lens, contact lens or intraocular lens for the test person for the above-given minimal use distance ause,min of the test person of approximately 33 cm would therefore amount to 2 dpt according to Reiner (or 1.5 dpt according to Schober).
In this example, the document is disposed during reading mostly at a use distance ause, that is, the reading distance of 40 cm to the eye of the test person (the reciprocal 1/ause of this is 2.5 dpt). For this reason, for a use accommodation ΔAuse of approximately 1 dpt (or 1.5 dpt according to Schober), an addition Add of 1.5 dpt (or 1 dpt according to Schober) would, in a satisfactory manner, satisfy the requirements of the test person. Because of the test person's residual maximum amplitude of accommodation ΔAmax (including depth of field T), the test person is able, at a reading distance ause of 33 cm, with an addition Add of only 1.5 dpt (or 1 dpt according to Schober) to see sharply. The addition Add of the spectacle lens, contact lens or intraocular lens therefore amounts to 1.5 dpt according to the theory of Reiner (or 1.5 dpt according to Schober) and should not exceed 2.0 dpt. The addition Add is optimally adapted to the requirements of the test person.
Unfortunately, in present day practice, mostly higher additions Add are prescribed which often leads to dissatisfaction of the wearer of the vision aid having the resulting lens design.
The following sections present an overview as to the most often used methods according to the applicants for determining the addition Add of a spectacle lens, contact lens or intraocular lens and the inadequacies of the method in each case.